1. Field of the Invention
The present invention relates to a method for continuously forming functional deposited films such as for a photovoltaic element and a deposition apparatus, and more particularly to a method for continuously forming functional deposited films such as for a large area photovoltaic element on a substrate and a deposition apparatus.
2. Related Background Art
Conventionally, deposited film forming apparatuses of three chamber separation type have been well known in which functional deposited films such as for a photovoltaic element are formed on a substrate by passing them consecutively through film forming chambers completely separated by a gate valve to form a semiconductor in order to improve the characteristics of the semiconductor. These film forming chambers completely separated were superior in the respect of preventing contamination with impurities but resulted in less productivity. Therefore, to form a semiconductor device such as a photovoltaic element on a strip-like substrate, a continuous film forming apparatus for continuously forming predetermined functional deposited films has been proposed in which the strip-like substrate is continuously conveyed from a strip-like substrate container to a plurality of reaction chambers in each of which a predetermined semiconductor film is formed thereon, and then received within another strip-like substrate container.
In Japanese Patent Application No. 62-36633, for example, a continuous plasma CVD method adopting the roll-to-roll method has been disclosed. According to this method, a photovoltaic element having semiconductor junctions can be continuously formed in which a plurality of glow discharge regions are provided, a longitudinally extending strip-like substrate of desired width is arranged along a passage passing through the plurality of glow discharge regions in sequence, and then a semiconductor of a desired conductivity type is deposited in each glow discharge region while the substrate is continuously conveyed in its longitudinal direction. In this Japanese Patent Publication, a gas gate is used for preventing the diffusion or contamination of impurity gases for the formation of a doped semiconductor layer into other glow discharge regions. Specifically, the plurality of glow discharge regions as above described are separated by a slit-like separation passage, and means is further provided for forming a flow of a scavenging gas such as Ar or H.sub.2 in the separation passage.
In this case, however, when a pressure difference is present from one film forming chamber to another in the gas gate, the problem arises that a film forming gas is prone to migrate from a higher pressure film forming chamber to a lower pressure film forming chamber.
To counter this problem, a conventionally adopted solution was such that the adjacent film forming chambers were equalized in pressure to eliminate such pressure difference, or the film forming chamber was placed at higher pressure to prevent the inflow of a gas from the adjacent film forming chamber, as disclosed in U.S. Pat. No. 4,438,723.
This apparatus is a continuous film forming apparatus for continuously forming semiconductor layers having p-, i-, or n-type conductivity (hereinafter referred to as a p-type layer, an i-type layer, and an n-type layer), for example, on a strip-like substrate, comprising a first reaction chamber for forming the p-type semiconductor layer, a second reaction chamber for forming the i-type semiconductor layer, and a third reaction chamber for forming the n-type semiconductor layer, wherein between the first and second reaction chambers, and between the second and third reaction chambers, separating means is provided for preventing an element constituting the p-type semiconductor layer and an element constituting the n-type semiconductor layer from mixing into the second reaction chamber, respectively, with the pressure of the second reaction chamber made higher than those of the first and third reaction chamber to effect the operation. The use of such continuous film forming apparatus allows a plurality of semiconductor layers having different compositions to be continuously deposited one over the other. The separating means is to isolate the adjacent reaction chamber from each other by providing a predetermined difference in internal pressure between the adjacent reaction chamber, thereby preventing the mutual diffusion of source gases for use in the reaction chamber. In this case, however, the separating means has a problem that there is a passage for the strip-like substrate provided through the adjacent reaction chamber, through which a source gas within the reaction chamber of the higher internal pressure inevitably mixes into the reaction chamber of the lower internal pressure, causing variations in the internal pressure within the latter reaction chamber, as well as in the plasma excited within the reaction chamber, so that it is difficult to form desired deposited films.
As a proposal for solving these problems in the continuous film forming apparatus as described in U.S. Pat. No. 4,438,723 as above cited, a continuous film forming apparatus having gas gate means has been disclosed in U.S. Pat. No. 4,462,332. The apparatus comprises a plurality of reaction chamber(i.e., a p-type semiconductor layer forming reaction chamber, an i-type semiconductor layer forming reaction chamber and an n-type semiconductor layer forming reaction chamber) wherein the gas gate means is provided between the adjacent reaction chambers, adjacent the i-type semiconductor layer forming reaction chamber. This gas gate means is to prevent the mutual diffusion of film forming source gases for use in the respective reaction chamber, having a structure such that a gate gas is introduced in only one direction to flow toward the p-type or n-type semiconductor layer forming reaction chamber. Also, the continuous film forming apparatus is provided with a magnet at an upper wall of the passage for the strip-like substrate through the gas gate means, whereby the strip-like substrate is brought into contact with the upper wall of the passage to reduce the size of the passage. This continuous film forming apparatus can solve some of the problems with the continuous film forming apparatus as described in U.S. Pat. No. 4,438,723, to some extent, but it is required to precisely control relevant parameters such as the conductance of the gas gate means or the flow rate of the gate gas to prevent the mutual diffusion of the source gases used across the adjacent reaction chamber. That is, for example, when a semiconductor photoelectric conversion element having a pin junction with high conversion efficiency is fabricated, it is necessary that the p-type and n-type semiconductor layers are relatively thin, and the i-type semiconductor layer is relatively thick, wherein each of the p-type and n-type semiconductors is formed by RF plasma CVD, for example, and the i-type semiconductor is formed by microwave plasma CVD which is capable of effecting fast film formation. In this case, the internal pressure of the i-type semiconductor forming reaction chamber during at the film formation is significantly lower than that of the p-type and n-type semiconductor forming reaction chambers during film formation. Therefore, it is required to prevent the inflow of impurities introducing source gases for use in the p-type and n-type semiconductor forming reaction chambers into the i-type semiconductor layer forming reaction chamber. However, this requirement is quite difficult to with the continuous film forming apparatus as described in U.S. Pat. No. 4,438,723. That is, in the continuous film forming apparatus as above described, the gas gate means is provided on either side of an i-type semiconductor forming chamber, causing the gate gas to flow via the gate gas means in the directions toward the p-type and n-type semiconductor forming reaction chambers. When the internal pressure of the i-type semiconductor forming reaction chamber is significantly lower than that of the p-type and n-type semiconductor forming reaction chambers, a reverse flow of a gas is caused, so that p- or n-type dopants containing source gas for use in the p-type or n-type semiconductor forming reaction chamber may mix into the i-type semiconductor forming reaction chamber. To solve this problem, a proposal has been made in Japanese Laid-Open Patent Application No. 3-30419 in which the gas gate means is provided centrally between the adjacent reaction chambers, so that the gate gas is introduced from the above and exhausted downward. A continuous film forming apparatus, according to this proposal, is comprised of a p-type semiconductor forming reaction chamber utilizing RF plasma CVD, an i-type semiconductor forming reaction chamber utilizing microwave plasma CVD, and an n-type semiconductor forming reaction chamber utilizing RF plasma CVD, wherein the gas gate means is provided centrally between the p-type semiconductor forming reaction chamber and the i-type semiconductor forming reaction chamber, and another gas gate means is provided centrally between the i-type semiconductor forming reaction chamber and the n-type semiconductor forming reaction chamber. However, in order that the gas gate means provided in the continuous film forming apparatus may sufficiently prevent the diffusion of source gases into the adjacent reaction chambers, it is necessary to decrease the conductance of a slit portion of the gas gate means, and/or increase the flow rate of the gate gas. However, in doing so, there is still left a problem to be solved. That is, the problem with the conductance of the slit portion is such that the conductance of the slit portion is governed by the shape of the slit portion so as to decrease in proportion to the length of the slit portion in the conveying direction of the strip-like substrate, and increases in proportion to the square of the height of the slit portion in the thickness direction of the strip-like substrate. Further, the size of the slit in the width direction of the strip-like substrate cannot be narrowed more than necessary due to the width of the strip-like substrate. Additionally, if the conductance of the slit portion is to be decreased, a problem arises that the strip-like substrate will inevitably vibrate or fluctuate in conveying the strip-like substrate. Therefore, in order to convey the strip-like substrate without contact of the film formation face with the wall face of the slit portion, it is necessary to provide a clearance of at least about 1 mm or more between the film formation face of the strip-like substrate and the wall face of the slit portion opposed to the film formation face. However, the provision of such clearance is naturally limited. Also, to decrease the conductance of the slit portion, it is conceived that the slit portion may be lengthened, but as the conductance only decreases in proportion to its length, the slit portion becomes quite longer, and the gas gate means becomes quite bulky so as to be impracticable. Also, the problem with the flow rate of the gate gas is such that when the flow rate of the gate gas is increased, the inflow rate of the gate gas into each reaction chamber correspondingly increases. In this case, a problem arises that the desired deposited film is difficult to form since variations are caused in the film forming conditions for each reaction chamber, including, for example, the internal pressure, the rate of dilution of the source gas, and the plasma state. To solve such problem, it is conceived that the exhausting ability of the vacuum pump used may be enhanced, but the vacuum pump may necessarily become larger in such event. Accordingly, this continuous film forming apparatus also has a problem to be improved in this respect.